to still be in equilibrium. (Ice has less tensile strength than rock, and is deformed at lower pressures and temperatures than rock.) The evidence is perhaps strongest for Ganymede, which has a magnetic field that indicates the fluid movement of electrically conducting material in its interior, though whether that fluid is a metallic core or a subsurface ocean is unknown. One of the mid-sized moons of Saturn (Rhea) may also be in equilibrium, as may a couple of the moons of Uranus (Titania and Oberon). However, the other ellipsoidal moons of Saturn (Mimas, Enceladus, Tethys, Dione and Iapetus) are no longer in equilibrium. In addition to not being in equilibrium, Mimas and Tethys have very low densities and it has been suggested that they may have non-negligible internal porosity, in which case they would not be satellite planets. The situation for Uranus's three smaller ellipsoidal moons (Umbriel, Ariel and Miranda) is unclear, as is that of Pluto's moon Charon. Eris' moon Dysnomia is larger than the three smallest ellipsoidal moons of Saturn and Uranus (Enceladus, Miranda, and Mimas), and must be quite massive to have tidally locked its parent; thus it has been included. Orcus' moon Vanth has been included as a possibility; it is larger than Mimas but is about the same size as non-ellipsoidal Proteus (Neptune VIII, the second-largest moon of Neptune, diameter 420±14 km). Also included is Varda's moon Ilmarë, which to within current uncertainties might be about the same size as Mimas. List – believed to be in equilibrium – confirmed not to be in equilibrium – uncertain evidence (Saturn VII is Hyperion, which is not gravitationally rounded; it is smaller than Mimas.) Titan has a denser atmosphere (1.4 bar) than Earth; it is the only known moon with a significant atmosphere. Triton (14 μbar), Io (1.9 bar), and Callisto (26 bar) have very thin atmospheres, but still enough to have collisions between atmospheric molecules. Other planetary-mass moons only have exospheres at most. Exospheres have been detected around Earth's Moon, Europa, Ganymede, Enceladus, Dione, and Rhea. An exosphere around Titania is a possibility, though it has not been confirmed. See also List of gravitationally rounded objects of the Solar System List of Solar System objects by size References Planetary science Planets Moons Lists of moons